A cryogenic sample changer

HAL Id: jpa-00245265

https://hal.archives-ouvertes.fr/jpa-00245265

Submitted on 1 Jan 1984

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

A cryogenic sample changer

The Sample Environment Group

To cite this version:

The Sample Environment Group. A cryogenic sample changer. Revue de Physique Appliquée, Société française de physique / EDP, 1984, 19 (9), pp.801-802. �10.1051/rphysap:01984001909080100�. �jpa- 00245265�

801

A cryogenic sample changer

The Sample Environment

Group

Neutron Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, ^U.K.

Résumé. ²⁰¹⁴ Un changeur d’échantillons a été mis au point ^{pour des} températures comprises ^{entre 30 K et 300} K,

en utilisant un cryostat à circuit fermé d’hélium. Le refroidissement est obtenu en améliorant la conductivité

thermique ^{entre les} porte-échantillons ^{en aluminium et} les barres de refroidissement en cuivre. On présente

l’ensemble de l’appareil ^et quelques ^résultats préliminaires.

Abstract. ²⁰¹⁴ A sample changer ^{has been} designed ^for cooling samples ^{to ca. 30 K} from ambient temperatures, using ^{a closed} cycle ^helium refrigerator ^to obtain the cryogenic temperatures. Cooling is achieved by producing good conductive contact between the aluminium sample holders and the copper cooling bars. The apparatus, and some preliminary results will be presented.

Revue Phys. Appl.19 (1984) ^{801-802 SEPTEMBRE} 1984,

Introduction.

The importance ^of

cryogenic

temperatures ^{in solid}

state materials research, ^{is well} established and is

expected

^{to continue on} the SNS. Some SNS neutron

spectrometers ^will

complete experiments

very

rapidly,

hence _any time spent

changing

samples ^becomes

wasteful. This will be

especially

^{true where} samples

cannot be cooled

by

^an

exchange

gas. Since radiant heat transfer is _very inefhcient at these temperatures the only viable alternative heat transfer _process is

conduction,

by

^{métal to métal contact.}

A

cryogenic

sample changer ^{has been}

developed

which is suitable over the temperature range 30 to 300 K. The sample ^holders

changing

mechanism and heat transfer _processes are

presented

^below.

1. The sample ^holders.

The

sample

holder consists of two

principle

parts;

the aluminium frame, ^{and the} top and bottom _copper contact

strips.

^The aluminium is used for its neutron

properties,

^{and the} copper

strips

^{are out} of the beam

area. The thermal

diffusivity

^of copper and aluminium is similar over the

cryogenic

temperature range. There- fore,

provided

^that

good

^{thermal contact is} established between them, ^{the two} metals will behave as a

sample

of a

single

metal. This is achieved in our

sample

^holder

by

friction welds

(although high

tensile steel bolts can

be used).

2. The sample changer.

The prototype sample

changer

^{for 10} samples ^uses

a closed

cycle refrigerator

^to

provide cooling

power.

Above the vacuum

tight

^{lid are a}

stepping

^motor,

which rotates a carousel

supporting

^the

samples ;

^and

pneumatic cylinders

^which

displace

^two

samples

^down.

The first

sample

^is

pushed

^{down onto a massive}

supporting

^{cold arm which is}

just

above the level of the beam, and the second

sample

^is

pushed

^{down into}

the beam _area, also

contacting

^{a cold arm. The first}

cold arm will take the

sample

^{down to ca. 90 K in}

about 15 min from room temperatures. ^{The second} cold arm

although

^not yet tested, ^is

designed

^for

operations

^{ca. 30 K.}

Sample moving speeds

^{are not}

a

limiting

factor. Thus the

stepping

^{motor can be}

driven one step ^{at a time}

by

^the

controlling

^micro-

processor. The ^most

important

characteristic of a

stepping

^motor under these conditions is its

pull-out

torque. ^A simple gear train

provides

^{a mechanical}

advantagé of 4 :

1, and backlash is avoided by ^allom ing only clockwise rotation of the carousel. The _pneuma- tic

cylinder

^{is double}

acting

^and

by controlling

^{the air}

flow a smooth sample

displacement

^{can be achieved.}

Since the axis of the push rod is concentric with the axis of rotation of the sample ^{in the}

scattering plane

it is

possible

^to obtain any sample angle

by rotating

the top

plate.

The

positions

^{of the} samples ^are determined

by

^the

use of opto switches and reed switches. The opto switch is closed when a sample is in the retracted

position.

The closure of this switch is tested before the

pneumatic cylinder

^is

opened

^or the carousel is rotated. The sample position ^is

interrogated

by ^the

controlling

microprocessor ^for consistency, any incon-

sistency ^{causes the} sample changer ^to stop with all the

samples ^retracted.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01984001909080100

802

The positions ^{of the} samples ^were measured for accuracy of location, in the Cartesian coordinate frame. The ^Z axis is vertical and the Y axis aligns along

a radius of the carousel. The individual sample posi-

tions were

reproduced

^{to an accuracy} better than the resolution of our gauge + ^{0.0125 mm, in the X- Y} plane. ^{In the Z} direction the sample relocation was

better than 0.5 mm. It is usual to relax the vertical

divergence

^{of neutron beams and so the}

discrepancies

between ^errors in the X-Y

plane

^{to those of Z is}

acceptable.

Assuming

Gaussian statistics the standard error on

the

replacement

^{of one} sample

by

any other is,

+ 0.07 ^mm. This error can be attributed to the sys- tematic errors inherent in _any mechanical construc- tion.

3. The heat transfer _process.

The most efficient heat transfer _process is

by

^electron

flow across the interface to the cold bars from the

sample ^{(another, less efficient} process, involves the flow of

phonons).

Because of the mechanical

strength

and

insulating properties

of aluminium oxide, ^contact

conductances (watts per ^meter

squared

per degree) ^of

aluminium are low.

Copper

^can

give

^reasonable

conductances but the best is obtained with gold.

Unfortunately gold plated

^aluminium samples ^have

very poor ^contact conductances because the

gold

^does

not adhere

directly

^to the aluminium. This

explains

the need for _copper

conducting strips

^{on the}

sample

frame. The _copper on the

sample

^{frame and on the}

cold arm are both

gold plated.

Further

improvements

ⁱⁿ the heat flow between metals can be obtained

by increasing,

the surface areas

in contact and the _pressures involved However, ⁱⁿ the context of

cryogenic engineering

very

high

pres-

sures are not

anticipated.

^{It is more}

profitable

^to

ensure flat contact between the

sample

and the cold

arm than to increase the contact pressure

(NB

^flatness

not smoothness

!).